Electronic components, such as semiconductor substrates or packaged semiconductor devices, for example Quad Flat No-Lead (QFN) packages and Chip-Scale Ball Grid Array (CSBGA) packages, undergo singulation into separate units after they have been packaged in an array arrangement. After sawing, the molded surfaces of the singulated electronic units are inevitably contaminated with sawing residue, such as saw dust and copper traces.
Typically, singulated electronic packages may be cleaned by mechanical agitation. In one prior art U.S. Pat. No. 6,446,354 entitled “Handler System for Cutting a Semiconductor Package Device”, a soft brush is used for creating mechanical agitation on the bottom molded surfaces of the packages. This action assists in removing the sawing residue by loosening the residue. The brush may be wet when wet brushing is required. Next, high pressure water jets are directed to the bottom molded surfaces of the packages to wash off and remove the loosened sawing residue.
There are disadvantages in using a brush for cleaning by mechanical agitation. For example, after a long period of use, some sawing residue is trapped in the brush. Periodic maintenance is therefore necessary to keep the brush clean, or otherwise the dirty brush may introduce dirt onto the surfaces to be cleaned instead. The cleaning effect of mechanical agitation will also be largely reduced with a dirty brush. Additionally, since there is actual contact with the molded surfaces of the packages during brushing, while the brushing force acting on the molded surfaces must be sufficiently large to loosen the sawing residue, the force should not be too large to dislodge the packages being held by vacuum on a pickhead during washing. Otherwise, missing singulated units may result. The problem of missing singulated units is more apparent for smaller packages, for instance QFN packages of dimension 3 mm×3 mm or smaller. Likewise, the pressure from the water jets for washing off the loosened sawing residue should not be too high to avoid dislodging the singulated electronic packages units.
To avoid mechanical agitation, U.S. Pat. No. 5,339,842 entitled “Method and Apparatus for Cleaning Objects” discloses the use of megasonic vibrations to enhance cleaning of electronic packages. Megasonic cleaning uses vibration frequencies of approximately 800 KHz to 1.8 MHz. Therefore, megasonic cleaning can be highly effective for removing particles having a particle size of about 1 micron or less. In this cleaning method, the bottom surface of a workpiece is cleaned by moving the workpiece over a first water tank overflowing with water such that the bottom surface is in contact with the surface of the running water. At the bottom of the water tank, a transducer generates megasonic waves that propagate through the water and upward to the surface of the water where the workpiece is moving through. The moving water and the megasonic waves loosen the sawing residue on the bottom surface of the workpiece, and the water carrying the loosened sawing residue flows into a second water tank surrounding the first water tank.
However, there are shortcomings with this method. Megasonic vibrations are high frequency waves which are highly focused in nature such that only a limited area of the workpiece can be cleaned. Additionally, some dirt or particles released from the surface of the workpiece may not be carried away to the second water tank since the water is flowing at a low speed. The loosened sawing residue may remain in the first water tank and return to adhere to the surface of the workpiece.
Therefore, it would be desirable to achieve a cleaning method for singulated electronic packages which sufficiently cleans the packages without damaging or loosening any singulated units.